1. Field of the Invention
The present relates to traveling wave tubes and more particularly concerns the helix of the slow wave structure of such tubes and a method of manufacture of such helix, or manufacture of other cylindrical objects.
2. Description of the Related Art
In travelling wave tubes, a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner that amplifies energy of the electromagnetic wave. To achieve desired interaction between the electron stream and the electromagnetic wave, the latter is propagated along a slow wave structure such as an electrically conductive helix that is wound about the path of the electron stream. The slow wave structure is conveniently explained as providing a path of propagation for the electromagnetic wave that is considerably longer than the straight axial length of the structure so that the traveling electromagnetic wave may be made to propagate axially at nearly the same velocity as the electron stream. More accurately described, the wave does not travel along the helix but travels along the axis of the helix at a speed much less than the speed of light in a vacuum because of boundary conditions imposed by the helix.
Slow wave structures of the helix type may be supported within a tubular housing by means of a plurality of longitudinally disposed dielectric rods that are circumferentially spaced about the slow wave helix structure. Various other means are available for supporting the helix within its envelope.
Typical slow wave structures of the prior art are disclosed in U.S. Pat. No. 3,670,196 to Burton H. Smith, U.S. Pat. No. 4,115,721 to Walter Fritz, U.S. Pat. No. 4,005,321 to Arthur E. Manoly, U.S. Pat. No. 4,229,676 to Arthur E. Manoly, U.S. Pat. No. 2,851,630 to Charles K. Birdsall, and U.S. Pat. No. 3,972,005 to John E. Nevins, Jr., et al.
The helix of the slow wave structure in the prior art is generally manufactured by winding or machining techniques. For winding a helix, a thin ribbon of an electrically conductive material may be wound around a mandrel and processed to properly shape the helix to the circular configuration of the mandrel. For machining a helix, a cylinder of helix metal may be cut into the desired pattern using electron discharge machining. Both winding and machining techniques are limited to manufacture of helices of relatively large size. Using such techniques, it is exceedingly difficult to fabricate small helices that are needed for higher frequency shorter wave tubes. For traveling wave tubes operating in the millimeter wave length, at frequencies above about 20 GHz, for example, circuit components including the helix are so small that conventional manufacturing techniques for the helix result in helices of poor dimensional precision. Moreover, yield of such processes is small because of the difficulty of handling and operating upon the very small parts. Thus, prior manufacturing techniques provide helices that are not dimensionally accurate, having poor tolerances, are not of sufficiently small diameter and have less dimensional stability, at least in part due to distortion arising from removal of the helix from its mandrel.
Although photolithographic techniques are used in semiconductor and flexible cable fabrication, these processes are employed on planar surfaces and have not been employed for the manufacture of components of traveling wave tube slow wave structures.
Accordingly, it is an object of this invention to provide hollow cylindrical objects, such as helices, and manufacturing methods therefor that avoid or minimize above mentioned problems.